It is desired to determine the cyanide (CN - ) concentration of a solution that

Question

It is desired to determine the cyanide (CN-) concentration of a solution that also contains CO32-.  The method being used to measure CN- also responds to CO32-, so it is critical to at least partially separate the ions so that CO is removed from CN.  A test solution, which is expected to be like actual samples, contains CN- and CO32- at concentrations 2.0 x 10-5 M and 8.5 x 10-4 M, respectively.  The goal in the separation is to be able to retain at least 99% of the CN- in the original solution while decreasing the concentration of CO32- to no more than 10% of the CN- concentration.  Using Ksp values listed in Appendix F of the text for Ag+, determine if this could be done.  Which anion would be precipitated out first upon addition of Ag+? How much of that anion would be left at the Ag+ concentration where the other anion would start precipitating?  Does this allow compliance with the requirements listed above?  Assume that you could re-dissolve a precipitated anion by adding acid.

Explanation / Answer

Ksp=[Ag+][CN-]

so, for AgCN,

[Ag+]*2*10^-5=2.2*10^-16

or [Ag+]=1.1*10^-11 M

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for Ag2CO3,

Ksp=[Ag+]^2[CO3 2-]

so,

[Ag]^2*8.5*10^-4=8.1*10^-12

or [Ag+]=9.76*10^-5 M

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so when Ag+ is added, Cn- will precipitate first and then CO3 2- willl precipitate.

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concentration of Ag+ when CO3 2- starts to precipitate=9.76*10^-5

so, the concentration of CN- will be x.so,

9.76*10^-5*x=2.2*10^-16

or x=2.25*10^-12 M=[CN-]

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for the third part, 99% of the CO3 2- =0.99*8.5*10^-4

=8.415*10^-4 M

so [Ag]^2*8.415*10^-4=8.1*10^-12

or [Ag]=9.81*10^-5

so, [CN-]*9.81*10^-5=2.2*10^-16

or [CN-]=2.24*10^-12 M

so yes this would indeed work since [CN-] is below 10%

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